Inadequate dopamine (DA) neurotransmission in brain neural pathways is a pathophysiological underpinning of many disabling neurologic and psychiatric illnesses including substance use disorders, attention deficit hyperactivity disorder and cognitive impairments in schizophrenia. The DA D1 receptor (D1R) is a G protein- coupled receptor (GPCR) identified as playing a central role in motor control, reward, attention and working memory. While activation of the D1R could provide a valuable therapeutic strategy for treating diverse brain disorders, undesirable properties of established catechol ligands have prevented therapeutic development for over 40 years. Our research group recently solved this catechol problem and reported the first non-catechol D1R selective agonists that have unprecedented drug-like properties. Many GPCRs, including the D1R, can signal not only through G proteins, but also via G protein-independent interactions with other signaling proteins including, most prominently, ?-arrestins. Unexpectedly, several of the novel non-catechol D1R agonists show biased signaling activity via G proteins, without engagement of ?-arrestins. This G protein biased signaling by novel D1R agonists may result in superior activation and/or reduced side effect profiles relative to unbiased D1R agonists, providing the innovative opportunity to fine tune D1R activity for neurotherapeutics. Historically, GPCR ligands have been optimized based on their potency, efficacy and specificity; however, another crucial parameter that impacts receptor signaling is the duration of ligand binding to the receptor (i.e., binding kinetics). We hypothesize that the duration of ligand binding is a key mechanism that determines signaling bias of selective D1R agonists towards G proteins versus ?-arrestin signaling. The goal of this research project is to validate and quantify the biased agonist activity of these novel D1R agonists and evaluate if faster ligand-receptor binding kinetics is a mechanism driving biased signaling. Aim 1 will use a combination of approaches including functional D1R signaling assays, competition and kinetic binding assays to define a scale of signaling bias for D1R agonists and the kinetic binding parameters (Koff, Kon) for unbiased and biased agonists. We will then correlate these kinetic binding parameters of agonists to ?-arrestin-mediated signaling outcomes of desensitization and internalization. Aim 2 will determine if faster binding kinetics drive non-catechol D1R agonist efficacy in vivo. We will assess kinetic binding parameters (Koff, Kon) for unbiased and biased non-catechol D1R agonists from mouse striatal membranes and compare the functional efficacy of D1R agonists on cocaine-induced locomotor behavior using wildtype and ?-arrestin2 knockout mice. This research project will expand our highly limited appreciation of ligand properties and mechanisms governing D1R biased signaling. If we discover that the duration of agonist binding is a mechanism underlying biased D1R signaling, we will provide a defining and measurable ligand property to aid in the design of biased agonists. This project will also validate the relevance and potential usefulness of G protein biased agonism for D1R function.